The present invention relates to apparatus for sensing fluid flow, and more particularly to sensing at high flow rates in a noisy environment, wherein the sensing is performed by transmitting and receiving ultrasonic signals along a path through the fluid. Such a high noise situation arises when a fast flowing fluid of low density is interrogated over a relatively long path, a situation encountered for example, when measuring the flow of stack gases. In the case of such flow, the level of noise arising in or carried by the surrounding wall, or the level of noise arising in the fluid itself, may each be large compared to the signal energy propagated and received through the fluid.
It often happens in a monitored process or condition, that the environment is hostile, inaccessible, or can be monitored only by sensors having a restricted range. All three of these limitations generally apply to stack measurements since the fluid may be at a temperature of several hundred degrees, must be interrogated by transducers mounted through the stack, often tens of meters above ground, and the transducers must be aimed at each other along a lengthy path which, although optimized for a particular expected fluid velocity, is subject to wandering as a fluid temperature, pressure and flow speed vary. When flow changes substantially, the interrogation beam becomes misaligned with the receiving transducer and signal quality drops.
By way of example, a typical stack gas sensing system may employ piezoelectric transducers mounted in vertically offset positions and aimed at each other from opposite sides of a three to ten meter diameter stack. Typically the transmitting and receiving transducers are of comparable size, and consist of a resonant electro-mechanical element mounted within a protective housing and coupled to a thin metallic diaphragm, about two to ten centimeters in diameter which directly transmits energy to the hot stack gases. The transducer assembly is mounted on a steel conduit that may extend into the stack, and may further include active fluid passages, for example, to carry a purge gas for preventing overheating of the transducer element.
Since the effective diaphragm area for coupling energy into or out of the rarified gas is small, and the path lengths are long, the level of the received signal of interest can be quite small compared to the level of noise generated elsewhere in the fluid, or reaching the transducer elements through the stack walls and transducer mounting structure. Moreover, the beam angle from the transmitting to the receiving transducer will generally change due to diffraction as the stack gas temperature varies, so even the gas-transmitted signal is subject to attenuation as the beam shifts.
Accordingly, it is desirable to enhance the overall signal quality in such systems.